<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="+" onclick="f(1)">
<input type="button" value="-" onclick="f(2)">
<input type="button" value="*" onclick="f(3)">
<input type="button" value="/" onclick="f(4)">
<div>
  结果:<span></span>
</div>
<script>
  function f(x){
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    if (isNaN(i1.value)||isNaN(i2.value)){
      s.innerText =" 输入错误!"
      return;
    }
    switch (x){
      case 1:
        s.innerText = i1.value*1+i2.value*1;
        break;
      case 2:
        s.innerText = i1.value-i2.value;
        break;
      case 3:
        s.innerText = i1.value*i2.value;
        break;
      case 4:
        s.innerText = i1.value/i2.value;
        break;
    }
  }
</script>
</body>
</html>